When fabricating an optical component such as a lens, it is very common to form a coating on a surface of the component, where the coating provides desired optical properties and/or physical properties. For example, the coating may provide an anti-reflective (AR) characteristic, a filtering characteristic, physical protection for the component, some other characteristic, or a combination of two or more characteristics. Coatings often include multiple layers of different materials that collectively provide the desired characteristic(s).
One known coating technique is to place a workpiece such as an optical component in a vacuum chamber with an evaporator. The evaporator generates a plume of coating material, which travels to and is deposited on a surface of the workpiece. Where the surface is relatively highly curved, for example highly concave or convex, it is not unusual for a given layer of the coating to have a peripheral region that is as much as 30% to 50% thinner than a central region of that layer, or even more than 50% thinner. This is due in part to the fact that, when a surface is highly curved, the plume of coating material will typically impinge on a central portion of the surface approximately perpendicular to the surface, and thus with a low angle of incidence, but will impinge on a peripheral portion of the same surface with a high angle of incidence. As a result, more coating material will be deposited on the central portion of the surface than on the peripheral portion. Consequently, the resulting layer of coating material will be significantly thinner in its peripheral region than in its central region.
In the case of an optical component, variations in the thickness of a coating layer can affect the optical performance of the coating. For example, if the coating is designed to pass light from a 1064 nm laser, it may do so properly in its central region, where the thicknesses are correct. But a 35% thickness variation in the peripheral region can cause a corresponding variation in the wavelengths passed in the peripheral region, such that the peripheral region passes wavelengths of about 676 nm to 709 nm, and not the intended wavelength of 1064 nm.
A further consideration is that different layers in the same coating often have different variations in thickness. For example, one layer may be 30% thinner in a peripheral region than in a central region, while another layer may be 50% thinner in a peripheral region than in a central region. Consequently, the ratios of thicknesses of different layers in the peripheral region of the coating can be different from the ratios of the thicknesses of those same layers in the central region.
Thus, even assuming that the layers all have the proper thicknesses and ratios of thickness in the central region of the coating, the thicknesses and the ratios of thicknesses in the peripheral region will typically not be correct. As a result, the coating may provide desired characteristics in the central region, but may fail to provide these desired characteristics in the peripheral region, or may at least exhibit a degradation of the desired characteristics in the peripheral region. Consequently, although pre-existing coating techniques have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.